Thursday, January 29, 2009

Above the critical altitude

First and foremost, all movements of the power controls on turbocharged engines should be slow and gentle. Aggressive and/or abrupt throttle movements increase the possibility of over boosting. The pilot should carefully monitor engine indications when making power changes.

When the waste gate is open, the turbocharged engine will react the same as a normally aspirated engine when the r.p.m. is varied. That is, when the r.p.m. is increased, the manifold pressure will decrease slightly. When the engine r.p.m. is decreased, the manifold pressure will increase slightly. However, when the waste gate is closed, manifold pressure variation with engine r.p.m. is just the opposite of the normally aspirated engine. An increase in engine r.p.m. will result in an increase in manifold pressure, and a decrease in engine r.p.m. will result in a decrease in manifold pressure.

Above the critical altitude, where the waste gate is closed, any change in airspeed will result in a corresponding change in manifold pressure. This is true because the increase in ram air pressure with an increase in airspeed is magnified by the compressor resulting in an increase in manifold pressure. The increase in manifold pressure creates a higher mass flow through the engine, causing higher turbine speeds and thus further increasing manifold pressure.

When running at high altitudes, aviation gasoline may tend to vaporize prior to reaching the cylinder. If this occurs in the portion of the fuel system between the fuel tank and the engine-driven fuel pump, an auxiliary positive pressure pump may be needed in the tank. Since engine-driven pumps pull fuel, they are easily vapor locked. A boost pump provides positive pressure—pushes the fuel—reducing the tendency to vaporize.

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